The present invention relates to agricultural tillage equipment; and more particularly, it relates to apparatus for achieving primary or secondary tillage using discs or "blades" as they are sometimes called. The type of tillage desired to be achieved by the present invention is similar to that achieved by conventional moldboard plows--that is, it is desired to uncover soil down to a depth of up to eight inches, and to break up the soil into larger clods and turn it over to bury any trash or residue that may have been left at the top of the soil. This is distinguished from another function sometimes performed by gangs of discs referred to as listers or bedders which are used primarily to prepare seed beds for planting with particular surface profiles depending on the crop. Such devices are not for primary tillage. In addition, tandem and offset disc harrows are used for seed bed preparation after primary or deep tillage has been achieved by a moldboard plow. Heavy disc tandems are also used to break up larger lumps of soil and pulverize the loose soil in the spring, where the soil might have been plowed using a moldboard plow in the previous fall.
Disc gangs or harrows are usually rotatably mounted on a common shaft which defines an angle relative to a line perpendicular to the direction of travel of the draft vehicle pulling it, called the working angle. Each disc gang includes a set of concave blades mounted on a common shaft. When the gang is operated at a right angle from the direction of travel, the blades roll over the ground like wheels with very little cutting. As the working angle increases disc rotation slows down, penetration increases depending on weight, and blades scoop and roll soil as they rotate. More soil is turned and trash coverage improves as the working angle increases. Soil pulverization is also increased, particularly at higher speeds, up to 7 mph. For disc harrows, whether tandem or offset, the working angle is normally in the range of 25.degree. to 30.degree..
An important aspect of disc gangs of this type is the spacing of the blades relative to the size of the blades used because this factor, together with the working angle, determines the amount of coverage a single blade will achieve. Disc gangs are designed so that each gang will achieve full soil coverage (that is, no soil will be left unworked after each gang passes). The spacing between adjacent discs is normally about 35-43 percent of the diameter of the disc, and rarely is it ever 50 percent of the diameter of the disc.
Conventional disc harrows normally employ a forward disc gang and a rear disc gang. Because of the size and narrow spacing of the blades and the working angle, each gang achieves a full working of the soil, as mentioned. That is to say, the forward gang, in the case of an offset arrangement, will have each blade throw the soil toward the right, whereas the rear gang will throw all of the soil to the left, so that no net lateral displacement of the soil is realized--that is not the case for a moldboard plow, for example. This has the further advantage that the side draft on the first gang is substantially offset by the side draft on the second gang, thereby making it easy to adjust the system to the draft of the tractor.
An "offset" system is one wherein all of the blades on the forward gang face one lateral direction, and all of the blades on the rear gang face the other lateral direction. Normally, the working angle of each gang may be the same, though provisions are made for independent adjustment, if desired. A "tandem" arrangement is one wherein the forward gang is divided into two sections, each facing a different lateral direction, so that the shafts on which the discs are mounted form a chevron shape. The rear gang, also divided into two side sections, has its sections facing the lateral direction opposite to that which the section immediately in front of it faces. Thus, the shafts for the rear gangs form an inverted chevron shape (when viewed from the top).
The discs or "blades" used in these systems may have a spherical shape or their working surfaces may be frusto-conical with a relatively flat or slightly domed center portion for mounting.
Disc harrows have some disadvantages. One disadvantage is that there is a double working of the soil. Although this has some advantages where it is desired to pulverize the soil, it uses more energy and is relatively inefficient in terms of working the soil. Further, because of the large number of blades relative to the width of a swath worked, in order to obtain soil penetration deeper than a few inches, it is a common practice to add weight to the frames, thereby further reducing efficiency and increasing the drag force. These systems are also limited in the forward speed at which they can be driven. Typically, the speed is in the range of 3 to 41/2 miles an hour. Modern tractors with higher drawbar power and better transmissions are capable of sustained speeds at higher levels, but the farmer usually cannot take advantage of this with conventional disc harrows because at higher speeds, the soil is thrown to the side a much further distance, and this increases the drag caused by the attachment. Further, at higher seeds the soil pulverization increases, and this is not desirable for primary tillage operations particularly in the fall. For example, the large slabs and clods created by a moldboard plow in the fall produce a soft and mellow soil in the spring after winter's freezing and thawing. If the soil were highly pulverized in the fall, the ground would become compact and hard over the winter with little or no capacity for absorbing and retaining water.
Disc harrows are also prone to "plugging"--that is, because of the close spacing of individual discs, an accumulation of crop residue such as corn stalks will become lodged between the disc, thereby greatly reducing their ability to work the soil. This characteristic is aggravated if the stalks are frozen or the gound is muddy. Another operational disadvantage of these systems is that whereas the first gang of discs will roll the top soil to cover the trash, the second gang, because it does substantially the same work, also rolls the soil and has a tendency to uncover trash that has been buried by the forward gang. A loss of efficiency occurs when trash is buried and then uncovered; and this also reduces the farmer's ability to determine the amount of trash that will ultimately remain buried.
There is a type of disc implement which is used for primary or secondary tillage. These are sometimes referred to as disc tillers or "one-ways" because they comprise a gang of sherical blades all facing the same direction, such as to the right. Hence, they normally throw the soil one-way and therefore require continuous plowing in the same direction, like a moldboard plow. One-ways are efficient tools in terms of horsepower-hours per acre for working large acreages, but they are known to be difficult to adjust because they operate at a substantially greater angle than disc tillers--usually about 45.degree.--and thereby create a substantial side draft which may upset any adjustment as soil conditions vary. For this reason, heavy tandem and offset disc harrows are sometimes used instead. Working depth of disc tillers may be varied from 3-8 inches, depending upon the size of the blade and the spacing, soil conditions and weight. Because there in only a single working of the soil at a relatively low speed, relatively deep furrows are left after the soil is worked.